Radar guidance system

ABSTRACT

Improved radar guidance system for the navigation apparatus of an aircrafts it flies over substantially flat terrain with random and different features. The system is generally made up of an airborne radar altimeter, a video signal processor and a master processor. The altimeter transmits a series of pulses at predetermined time intervals for impacting a plurality of spaced points along the aircraft ground track. The echo signal of each pulse-impacted point processor divides the amplified signal into corresponding signals. A comparator of the signal processor transforms one of the divided signals into a constant output. A track and hold arrangement of the signal processor correlates the delayed leading edge of the comparator output with the other of the divided signals so as to determine a point of intersection therebetween. The intersection point is a quantitized value of the delay sample of a given echo signal that is indicative of the average weighted reflectivity value of a preselected annulus of one or more random and different features about a pulse-impacted track point. The master processor combines a series of delay samples into a sample data array and then compares the sample array with a series of data arrays of a stored data matrix at a given altitude of the aircraft for the purpose of determining the aircraft flight path. Depending on the requirements of the signal processor, a Doppler filter may be used.

This invention concerns a radar guidance system for an airborne vehicleand more particularly it relates to an improved radar guidance systemfor use with a navigational apparatus of an airborne vehicle so as toenable the determination of the correct flight path or terminal fix ofthe vehicle during flight over substantially flat terrain.

BACKGROUND OF THE INVENTION

Various guidance systems have been designed in the past for assisting anairborne vehicle in determining if it is following its intended flightpath and if not to provide a correctional signal to the navigationapparatus. For example, U.S. Pat. No. 3,328,795 to Hallmark discloses aguidance system for ascertaining the flight path of an airborne vehicleover hilly terrain. The guidance system is provided with an altimeterarrangement for determining the difference in elevation from one radarpulsed area of the hilly terrain to another along the ground track ofthe vehicle. The computer of the system compares the reflected signalsof the different pulsed areas of the hilly terrain with its data base inorder to ascertain the vehicle flight path. U.S. Pat. No. 4,144,571 toWebber relates to another airborne vehicle guidance system for use onlyon hilly terrain. The system is generally made up of a processor forreceiving data from storage, and also for receiving data of a radarsample of hilly terrain for indicating aircraft elevation. The processorincludes a Kalman filter for resolving the aforesaid data into anupdated three coordinate airborne position of the aircraft prior to theguidance system taking another radar sample of a hilly terrain sectionfor another updated position of the aircraft along its flight path. U.S.Pat. No. 4,495,580 to Kearns discloses an airborne automated navigationsystem for automatically monitoring the position and velocity of anaircraft along its flight path while at the same time correcting theposition of the aircraft when it is not along its intended path. Thesystem is generally made of a radar altimeter, a navigation device, anda stored reference map. When sampled outputs of the navigation deviceand altimeter are correlated with the stored map, and such correlation,e.g., indicates that the aircraft is deviating from its intended path, adata register output of the system transmits a correction signal outputto the navigation device so as to direct the aircraft flight control tofollow the correct course. However, none of the aforediscussedreferences whether taken alone or in any combination remotely suggestthe improved aircraft guidance system of the invention for controllingthe operation of a navigation apparatus in maintaining the aircraftalong its intended flight path (especially at a terminal fix) and eventhough the aircraft is flying over substantially flat terrain withrandom and different features therealong. The system is generally madeup of a radar altimeter, a video signal processor and a master processorconnected to both the altimeter and the signal processor. The altimetertransmits a series of pulses at predetermined time intervals forimpacting a plurality of relatively spaced points along the ground trackof the aircraft vehicle. The signal processor receives the echosignature of each radar-pulse impacted ground track point and isprovided with means for dividing each signature into first and secondsignals. One of the divided signals is transformed and delayed and thenrecombined with the other divided signal so as to determine anintersection point between the signals. This point is a quantitizedvalue of a given signature that indicates the average weightedreflectivity value of an annulus about a pulse-impacted ground-trackpoint. The quantitized value of each signature is then transformed bythe signal processor into digital form. The master processor receivesthe digital signal of each signature and combines same into a series ofdigital signals that represents a sample data array. The sample array isthen compared with a series of digital arrays of a stored matrix for thepurpose of substantially matching the sample array with one of thestored arrays thereby enabling an accurate determination of the vehicleflight path.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved radar guidancesystem for aircraft and the like over substantially flat terrain havingrandom and different features therealong.

Another object of the invention is to provide an improved radar guidancesystem for aircraft that can be readily inserted into existing aircraftradar guidance systems for enhancing the usefulness and sensitivitythereof.

Still another object of the invention is to provide an improved aircraftradar guidance system for transforming a plurality of radar altimeterecho signals into a series of delay line samples for the purpose ofminimizing bandwidth requirements of the system while at the same timeenabling the construction of a compact and effective guidance system.

In summary, the improved aircraft radar guidance system is generallymade up of a radar altimeter, a video signal processor, and a masterprocessor that is separately connected to both the signal processor andthe altimeter. The navigation apparatus of the aircraft during flighttransmits a signal to the master processor for initiation of theguidance system such as, e.g., when a terminal fix position is requiredby a missile prior to striking a target. At this time the masterprocessor transmits a signal to the radar altimeter for transmitting aseries of radar pulses at predetermined time intervals for impacting aplurality of relatively spaced points along the aircraft ground track ofsubstantially flat terrain having one or more random and differentfeatures therealong. Each pulse, as initially transmitted from thealtimeter, is preferably of very short or compressed time duration.

The echo signal about each pulse-impacted ground-track point istransmitted by the altimeter to a logarithmic amplifier of the videosignal processor. The output of the amplifier is divided into twosignals. One of the divided signals is directed to the positive input ofa comparator of the signal processor. The master processor is seriesinterconnected to a threshold register and a digital-to-analogconverter. The output of the converter is connected to the negative biasinput of the comparator. The master processor during an operative modeof the guidance system and prior to the master processor transmitting acommand signal to the altimeter to transmit a series of radar pulses,functions to determine the noise threshold level as the aircraftadvances along its flight path. When the noise threshold level isdetermined by the master processor, the register transmits a signal tothe comparator for substantially removing the noise level from eachsignal received by its positive input. The output of the comparatortransforms each divided echo signal received into a noise-free, constantoutput.

The signal processor is also provided with a delay line device and atrack-and-hold arrangement. The device is connected to the output of thecomparator and serves to delay the leading edge of each transformedcomparator output of any amplified echo signal received by the system.The track-and-hold arrangement is separately connected to the delay linedevice output and the other divided output of the amplifier. Thearrangement functions to correlate the output of the other dividedsignal of a given echo signature with its associated transformed anddelayed output of the one divided signal. This correlation enables thedetermination of a point of intersection between the outputs at theleading edge of the one transformed and delayed output. The point ofintersection that is determined for each echo signature is indicative ofa quantitized value of a delay-sample annulus of average-weightedreflectivity value of one or more random and different terrain featuresabout a pulse-impacted ground-track point. The quantitized value outputof the arrangement for the correlated and transformed signals of eachecho signal received is directed to an analog-to-digital (A/D) converterof the signal processor.

The master processor receives the digital output from the A/D converterof each quantitized value of an echo signal and combines all of thedigital outputs received into a series of digital samples that form asingle delay sample data array for each ground track radar pulseoperative mode of the guidance system. The master processor then furtherfunctions to compare the delay sample array with a series of stored dataarrays of a stored matrix at a given altitude for the aircraft flight sothat the processor is able to accurately determine the location of theground track path as previously sensed by a series of pulses from theguidance system of the invention. If the actual ground track path doesnot correspond to the intended ground track path of the vehicle, themaster processor transmits a signal to the navigation apparatus forcorrecting the flight path of the vehicle.

Depending on the requirements of the guidance system, a Doppler filtermay be incorporated or connected to the output of the logarithmicamplifier of the signal processor. During operation, the filter willremove all unit areas of an amplified echo signature exceptdiametrically opposed areas of the echo signature in relation to apulse-impacted ground-track point. These diametrically-opposed areas aregenerally perpendicular to the ground track of the aircraft. One of theadvantages of the Doppler filter is that it assures a more definitivedelay line sample of an echo signature for analysis without sacrificingsensitivity of the improved guidance system.

The term "quantization" or "quantitized value" as used throughout thespecification and claims specifically refers to a common point betweenan echo signature waveform that stems from a radar pulse-impacted pointalong the ground track of an airborne vehicle and a correspondingwaveform extracted or divided-out from the echo signature waveform. Theextracted waveform is transformed into a constant output. The leadingedge of the constant output is then delayed a predetermined amount so asto enable the formation of a common point between the two waveforms thatis indicative of a value which is of assistance in determining theground track of an airborne vehicle, e.g., the average-weightedreflectivity value of a delay sample annulus about the radarpulse-impacted point along the ground track of the airborne vehicle.

Other objects and advantages of the invention will become more apparentwhen taken in conjunction with the following description of theinvention together with the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of an operative embodiment of the improvedairborne radar guidance system of the invention.

FIG. 1A is an enlarged, fragmented, diagrammatic view of FIG. 1 withparts added, and illustrates further details of the invention.

FIG. 2 is a diagrammatic view of various electronic components of theinvention.

FIG. 3 is a diagrammatic view similar to FIG. 2, but illustratingfurther details of the invention.

FIG. 4 is a diagrammatic view of a flow chart of an operative embodimentof the guidance system for the purpose of determining the noisethreshold level of the environment in relation to the guidance system ofthe invention during aircraft use.

FIG. 5 is a cartographic view of a flat terrain section and such viewhas superimposed thereon a multicell grid pattern of the sectiontogether with the actual and intended ground tracks in dotted and solidlines of an airborne vehicle wherein the dotted line ground track isprovided with a series of spaced points and a concentric ring about eachpoint of the series that are indicative of some of the areas of impactof a series of relatively spaced radar pulses from the improved guidancesystem on the vehicle.

FIG. 6 is a schematic view of an echo signal signature in relation totime as received from a radar-pulse-impacted area about a ground trackpoint of substantially flat terrain and further illustrates an analogquantity of constant value as extracted from the echo signal at apredetermined point in time and in relation thereto.

FIG. 6A is a schematic view of the comparator output in relation to theecho signal of FIG. 6.

FIG. 6B is a schematic view of the delay line output and illustrates indotted lines the relationship of the delay line output with the signalsof FIGS. 6 and 6A.

FIGS. 7 and 7A are diagrammatic views of another flow chart of a radarpulse operative embodiment of the guidance system.

FIG. 8 is a diagrammatic view that indicates a single array of a seriesof digitized sample values as obtained from a plurality of echosignatures during aircraft use of the invention.

FIG. 9 is a schematic view of the multicell grid pattern for the terrainsection of FIG. 4 together with the stored and predeterminedaverage-weighted reflectivity value of each cell wherein thereflectivity value of each cell is indicative of a preselected delaysample annulus area thereabout at a given altitude of the aircraft.

FIG. 10 is a table that illustrates a technique for determiningreflectivity values of certain cells of the grid of FIG. 9.

FIG. 11 is a schematic view of another embodiment of the invention thatutilizes a delay Doppler sampling technique.

FIG. 12 is a diagrammatic view of a video signal processor with partsremoved and other parts added and illustrates details of the otherembodiment of FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

With further reference to FIG. 1, an aircraft 10, such as, e.g., apilotless missile, is flying along its flight path 12 at a desiredaltitude above a substantially flat terrain 14 having one or more randomand different features. These random and different features are, forpurposes of illustration, a series of two forested areas 16, a field 18,a body of water 20 and an asphalt-covered road 22. Because of thevarious forces encountered by aircraft 10, its actual flight path aftera flight period is usually a deviation from the intended vehicle path.Depending on the importance of maintaining the intended path, animproved guidance system 24 is provided for maintaining the desiredflight path over substantially flat terrain as will now be described.

It is further evident from FIGS. 1 and 1A, that for analyzing thecapability of system 24, a radius line R can be drawn from antenna 29 tothe outer circumference of annulus 90. Then a perpendicular line DE isprojected from the lower end of R so as to intersect the innercircumference of annulus 90. As the result of projecting line DE, aright triangle BDE is formed that is similar to right triangle BCA.Since altitude "h" of aircraft 10 is known, both range resolution Δr andradial extent ΔR of annulus 90 can be readily determined. Determiningparameters Δr and ΔR are important in designing the components of system24 so as to provide accurate analysis of relatively spaced and sensedannular areas of random flat terrain in determining the flight path ofaircraft 10.

As illustrated in FIG. 2, system 24 is generally made up of a masterprocessor 26, a radar altimeter 28, and a video signal processor 30.Processor 26 can be any suitable commercial device such as, e.g., amicroprocessor furnished by National Semiconductor, Inc. and designatedIMP-16C. A memory 32 has stored therein cartographic map data ofsubstantially flat terrain with random and different features over whichaircraft 10 is to fly. The data is made up in matrices of more than onesection of the terrain where each matrix or grid is generally made up ofa series of data arrays. The series of arrays of a matrix is for a givenaltitude of the aircraft and relates to a series of average-weightedreflectivity values of a particular area of flat terrain 14 with thearea usually having one or more random and different features as will bemore fully explained hereinafter. A real time clock 34 is connected toprocessor 26. An inertial navigation apparatus 35 of aircraft 10 is alsoconnected to processor 26. Altimeter 28 is provided with aconical-shaped downward-looking antenna 31 for transmitting a series ofradar pulses with each pulse 33 of the series being preferably ofcompressed time duration and teardrop-like configuration as shown inFIG. 1. The altimeter also includes an inverted coaxial dish-shapedantenna 37 for receiving each echo signature or signal of a radar pulseand/or noise.

As best shown in FIG. 3, signal processor 30 is generally made up of alogarithmic amplifier 36, a comparator 38, a delay line device 40 and atrack-and-hold (T/H) arrangement 42. A suitable commercial device forT/H arrangement 42 is furnished by Analog Devices, Inc., and isdesignated HTS-0100. The output of amplifier 36 is parallel connected tothe analog input of comparator 38 and, via lead 43, to the analog inputof the T/H arrangement. The signal processor is also generally made upof a threshold register 44 and a digital-to-analog (D/A) converter 46.Another output of the master processor via lead 48 is connected to theinput of register 44. Register 44 and converter 46 are seriesinterconnected to the negative input of comparator 38. Register 44, vialead 50, is feed-back connected to processor 26. The output ofcomparator 38, via lead 49, is connected to the input of delay device40. The signal processor also includes an analog-to-digital (A/D)converter 52 that is connected to the output of T/H arrangement 42. Theoutput of converter 52 is connected to processor 26.

Navigation apparatus 35 of aircraft is connected via its output 56 toprocessor 26. Another output 58 of processor 26 is connected to thenavigation apparatus.

Prior to a series of any suitable number of radar pulses beingtransmitted by altimeter 28, such as, e.g., a series of "N" radarpulses, in order to locate the actual flight path of vehicle 10,guidance system 24 initially and advantageously functions to determinethe threshold noise level of the environment in the vicinity of theaircraft. With reference to the flowchart of FIG. 4, a signal is sent bynavigation apparatus 35 to start processor 26. To this end, theprocessor, via lead 48, initially sets a threshold value for register 44as confirmed by feedback 50. The threshold value is converted to analogform by converter 46 and transmitted to the negative input of comparator38. The comparator also receives the amplified analog noise signal fromamplifier 36 as initially received by altimeter receiver antenna 37.Assuming that the processor-set threshold value is below the level ofthe amplified noise signal received by altimeter antenna 37, the outputof comparator 38 is delayed by element 40. Element 42 correlates thedelayed output of element 40 with the amplified noise output ofamplifier 36. Such correlation results in a point of intersectionbetween the correlated signals. This point determines the quantitizedvalue which is digitized by converter 52. The output of converter 52will indicate noise when tested by tester 62, and then detected bydetector 64. Element 66 will then instruct processor 26 to raise thethreshold value for register 44. Assuming that the next-set-thresholdvalue of register 44 by processor 26 is above the level of the noisesignal received by altimeter 28, the output of comparator 38 will bezero, and the noise signal detected by element 64 will be zero.Consequently, detector 64 will transmit an output, via element 68, tolower the threshold value that is set by processor 26 for register 44.Thus, it is evident that processor 26 of system 10 repeats itself againand again until the best threshold value is found for register 44 inrelation to the noise received by altimeter 28 during flight of aircraft10 so as to substantially eliminate noise from an echo signature whensystem 10 is being operated in the radar pulse mode as will be morefully explained hereinafter.

As indicated in FIG. 5, terrain 14 has superimposed thereon a gridpattern 70 of a series of uniform square sections or cells 72 so as todivide up the terrain into a rectangular coordinate system of five (5)columns and eight (8) rows of adjoining and interconnected cells. Asfurther shown in FIG. 5, the actual ground track of aircraft 10 alongterrain 14 as will be sensed by a series of radar pulses from system 10is depicted by dotted arrowed line 74. On the other hand, the intendedground track of aircraft 10 along terrain 14 is indicated by aparallel-spaced arrowed solid line 76. When system 10 transmits a radarpulse 33 from its antenna 30, the pulse propagates from the antennauntil it impacts terrain 14 at a predetermined point 80 as shown inFIG. 1. By reason of the continued outward expansion of pulse 33 as itpropagates from altimeter antenna 31 at the speed of light, it impactsterrain 14 in an ever-enlarging concentric circle about point 80 untilthe pulse dissipates. Since pulse 33 impacts terrain 14 at point 80 thatincludes one part of field 18 as well as impacts in ever-expandingconcentric annuli about point 80 that includes other parts of field 18,water 20, road 22 or any combination thereof, the reflected analog echosignature of pulse 33 for a known altitude of aircraft 10 will indicateat any ground radial point from point 80 the average-weightedreflectivity value of various portions of the field, water and roadthereabout.

Since a series of radar pulses at predetermined time intervals arerequired to be transmitted by system 24 for impacting terrain 14 so asto determine ground track 74 of aircraft 10, clock 34 functions to driveprocessor 26 in order that each radar pulse 27 will impact terrain 14 ata point that corresponds to the center of a grid cell 72 of grid 70 asstored in memory 32. For purposes of illustration, system 24 will bedescribed in a radar-pulse operative mode where a series of five radarpulses are transmitted at predetermined time intervals that impactterrain 14 at a corresponding series of initial impact points 80, 82,84, 86 and 88 as illustrated in FIG. 5. Each impact point 80, 82, 84, 86or 88 corresponds to the center of a grid cell 70 with which it isassociated. Moreover, it is to be understood that, depending upon therequirements for accurately determining the actual ground track ofaircraft 10 and the time permitted for making such determination, anysuitable number of radar pulses can be transmitted by altimeter 28during use of system 24. In other words, the greater number of pulsesfor impacting terrain 14 provides greater accuracy in determining theground track of aircraft 10.

As further evident from FIG. 1, when radar pulse 33 is transmitted fromaltimeter 28, it, for all practical purposes, expands radially anduniformly outward from about nadir or altitude line "h". When theexpanded pulse impacts terrain 14 about point 80 the outer radial extentof the impacted pulse at any point in time can be indicated by a radialline "R" extending from antenna 31 to a point on the outer extent of thesignal as it continues to impact, with respect to time, an increasingarea of terrain 14 about point 80. The angle between divergent lines "h"and "R" is designated angle "θ". In order to minimize bandwidthrequirements of system 24 in analyzing the echo signature of each pulseafter impacting terrain 14 at a preselected point, e.g., point 80, anouter annulus 90 of radar-pulse-impacted terrain 14 about point 80 isselected, such as, e.g., the outer annulus of nominal width where itscenter circumference about point 80 is intersected at one point by theouter end of radial line "R". As will become more apparent hereinafter,one of the reasons for selecting outer annulus 90 of the echo signatureabout point 80 is that the electronic characteristics of system 24 willfunction with a minimal bandwidth when the range resolution is about tenfeet and the delay sample time is about fifty nanoseconds (50 ns). Thus,the remaining initial pulse-impacted terrain points 82, 84, 86, or 88 ofthe remaining series of four subsequent pulses 33 by altimeter 28 iseach provided with its annulus 92, 94, 96 or 98 that geometricallycorresponds to annulus 90 as shown in FIG. 5. The average weightedreflectivity value of each annulus 90, 92, 94, 96 or 98 is analyzed bysystem 24 in determining the ground track of aircraft 10.

An analog echo signature 100 with respect to time of each pulse impactedarea about a terrain point, e.g., 80, in FIG. 1, is usually of the formshown in FIG. 6 for a known aircraft altitude. The initial part of thesignature corresponds to noise as indicated by undulating portion 102 ofsignature 100. As the result of the operative mode of FIG. 4 of system24 for determining threshold noise level, threshold level has beendetermined for signature 100 as indicated by dotted line 104 in FIG. 6.Depending on the average-weighted reflectivity values of random anddifferent features about a radar pulse-impacted terrain point; it is tobe understood that the downward slope of signature 100, which isrepresentative of the reflectivity values of the outer annuli about agiven point, (e.g., point 80 of FIG. 1) can be higher or lower than thatshown in FIG. 6.

In a radar-pulse operative mode of system 24, processor 26 in responseto a command signal from apparatus 35 will cause altimeter 28 totransmit a series of radar pulses at predetermined time intervals inaccordance with the output of clock 34 as indicated by elements 108 inthe flow chart of FIG. 7. The analog echo signature output of a givenpulse-impacted terrain point of amplifier 36 is divided between theforward bias input of comparator 38 and the analog input of T/Harrangement 42. By reason of the predetermined threshold value ofregister 44, comparator 38 provides a constant output above noise levelas depicted by signal 110 in FIG. 6A. The leading edge 112 of signal 110corresponds to threshold point 104 of FIG. 6. As result of comparatoroutput signal 110 being subjected to the action of device 40, itsleading edge 112 is delayed a predetermined amount of time as indicatedby delayed signal 114 with its leading edge 116 as illustrated in FIG.6B. The predetermined time delay as indicated by arrowed line 118 inFIG. 6B, of each delayed output signal 114 of an echo signature to beanalyzed substantially corresponds to the central radius of an annulusof reflectivity to be analyzed by system 24 about a radar-pulse-impactedpoint of terrain 14. T/H arrangement 42 functions to correlate the otherdivided analog output of an echo signature of amplifier 36 with thedelayed and transformed comparator output of the echo signature bydevice 40. As the result of this correlation and as further indicated byFIG. 6, an intersection point 120 is determined between analog anddelayed signals 100 and 114 respectively. This point 120 is aquantitized value that is proportional to the average weightedreflectivity of annulus 90 about a pulse-impacted point 80 of terrain14.

Since output 124 of T/H arrangement 42 is of a constant value andincludes point 120, it is adventageously transformed into digital formby converter 52 as illustrated in FIGS. 6 and 7. Since the digitaloutput of each echo signature from converter 52 corresponds to a senseddigital value of an annulus of reflectivity about a pulse-impactedterrain point; the sensed value is stored by element 126 as a senseddelay sample of a profile sensed array. When a predetermined number ofsensed samples are stored by element 126, the "yes" output of element128 will indicate that no further radar pulses will have to betransmitted by altimeter 28 as commanded by element 130 to processor 26in accordance with the predetermined time intervals of clock 34 in themanner shown in FIGS. 7-7A.

For purposes of illustration, it is assumed that a series of five (5)radar pulses are to be transmitted by altimeter 28 so as to impact aseries of five terrain points 80, 82, 84, 86 and 88 as shown in FIG. 6.Each point 80, 82, 84, 86 and 88 corresponds to the center of anadjoining cell 72 along intermediate or third column of grid 70. Theseries of five impacted points indicate results in the formation of line74 that indicates the actual ground track of aircraft 10. Since thespace between adjoining points 80 and 82, 82 and 84, 84 and 86 or 86 and88 corresponds to a predetermined and uniform time interval of clock 34,it is to be understood that the time interval can be selected so anydesired space exists between adjoining points 80 and 82, such as, e.g.,several hundred feet. Similarly, instead of a series of five radarpulses to determine the actual ground track of aircraft 10 at a givenaltitude, element 128 in the flow chart of FIGS. 7-7A can be arranged tofunction so as to provide any number of sample values for a sensed arraysuch as, e.g., sixty samples in response to sixty radar pulses so as tomore accurately determine ground track 74 of aircraft 10.

Assuming that element 128 has obtained a sample array of five annuli ofreflectivity values of a series of five points as indicated by profilesample 132 at a given altitude of aircraft 10, this sample is nowmatched and correlated with grid pattern (or matrix) 70 of predeterminedreflectivity values at the same altitude as shown in FIGS. 8 and 9.Since terrain 14 has random and different features and since theselected annulus of reflectivity about each point has a radius greaterthan the length or width of each cell 72, the cells of grid 70 havedifferent values of reflectivity as representative of each annulus aboutthe center point of a given cell as depicted in the matrix of FIG. 9. Asdepicted in FIG. 10, table 134 is representative of one technique fordetermining the average weighted reflectivity value of an annulus aboutthe pulse-impacted point of a grid cell 72. As shown by table 134 thedifferent and random features of terrain 14 are assigned differentreflectivity values. Then for the preselected ring or annulus about cell13 that is related to pulse-impacted center point 80 of FIG. 5, it isevident that the ring encompasses thirty-three percent (33%) of forestedarea 16, sixty-three percent (63%) of field 18, zero percent (0%) ofwater 20 and four percent (4%) of asphalt road 22. Then multiplying thepercentage of each feature with its assigned and predeterminedreflectivity value for annulus 90, the average-weighted reflectivityvalue is determined. In other words, 33% times 2.0+63% times 7+4% times24.0=0.66+4.41+0.96≅6.0 for cell 13 that corresponds to annulus 90 assensed by system 10. In similar fashion, the average weightedreflectivity value is determined for cell 18, 23, 28 or 33 thatcorresponds to sensed annulus 92, 94, 96 or 98 respectively. As evidentfrom FIGS. 8-10, the sensed values of reflectivity of rings 90, 92, 94,96 and 98 is slightly different than the predetermined calculated valuesfor cells 13, 18, 23, 28 and 33 of grid 70. One of the reasons for thisdifference is that the reflectivity values for the different features ofterrain vary from day to day such as when they were first calculatedfrom preselected data in forming reference grid 70 prior to flight ofaircraft 10 and then subsequently the reflectivity values of terrain 14were sensed and electronically analyzed by system 24 as aforedescribedduring flight of aircraft 10.

When element 128 indicates that a sensed sample array 132 having aseries of five delay samples has been obtained by system 24 during itsuse, then the "yes" output of element 128 commands element 134 to searchfor matrix or grid 70 at the known altitude of aircraft 10 having a dataarray that is substantially matched by the sensed profile as indicatedin FIGS. 7-7A, and 8-9. It is evident from shadowed area 136 of FIG. 9that cells 13, 18, 23, 28 and 33 are substantially matched by sensedprofile 132.

Since shadowed area 136 of matrix 70 indicates actual ground track 74 ofaircraft 10; processor 26 then functions as indicated by element 138 inFIG. 7A to determine the difference, if any, between intended and actualground tracks 74 and 76. Then, if a difference exists between tracks 74and 76 as shown in FIG. 5, element 140 of FIG. 7A causes processor 26 totransmit a signal to apparatus 54 for correcting the flight path ofaircraft 10 prior to its destination or striking a target.

It is noted here that memory 32 is provided with a plurality of storedmatrices (data base) where each matrix of the plurality has a grid cellpattern similar to the cell pattern of grid 70 and each matrixcorresponds to a given but different altitude. And of course any cell ofeach stored matrix for a given aircraft altitude has a reflectivityvalue that is different from the reflectivity value of a correspondingcell of any other stored matrix for a different aircraft altitude. Inother words, the reflectivity of a pulse-impacted terrain point is afunction of altitude as well as the random and different featuresthereabout. Matrix 70 indicates the calculated reflectivity value of thepreselected annulus about each cell when the aircraft was at an altitudeof 20,000 feet.

In one reduction to practice of system 24, it operated at a minimumbandwidth (B) of about fifty megahertz (50 MHz) and the system radarrange resolution (Δr) is about ten feet (10 ft). As evident from FIGS. 1and 1A, triangles ACB and EDB are similar right triangles. DB oftriangle EDB corresponds to Δr. With a B of 50 MHz and Δr of 10.0 ft itcan be mathematically shown that ΔR is two hundred feet (200.0 ft) whenthe altitude (h) of aircraft 10 is twenty thousand feet (20,000 ft).With this altitude, delay line 40 functions to delay leading edge 116 ofcurve 114, fifty nanoseconds (50.0 ns) as shown in FIG. 6B.

Another embodiment of the guidance system of the invention isillustrated in FIG. 11. The video signal processor of guidance system24' is provided with a Doppler filter 142 that is interposed between andserially interconnected to amplifier 36 and comparator 42 as shown inFIG. 12. By virtue of the Doppler filter all unit areas of the echosignal about a pulse-impacted ground point, e.g., point 80, are removedexcept for diametrically opposed unit areas of the echo signal that aredisposed at generally right angles to ground track 76 of aircraft 10 asdepicted in FIG. 11. Then, when sample device extracts a delay samplefrom the Doppler filtered echo signal, it will only includediametrically opposed unit areas within a ring 90 of the echo signal ata predetermined time interval. Although the filter reduces the magnitudeof the echo signal and the sample extracted therefrom, it provides asample with less noise effects and thus enables guidance system 24' tohave greater sensitivity in matching a sensed profile with a matrix ofmemory 32 and a data array thereof.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced than as otherwise specifically described.

What is claimed is:
 1. A radar guidance system for the navigationapparatus of an airborne vehicle, said system comprising:radar altimetermeans for transmitting a series of pulses for impacting a series ofrelatively spaced points along the vehicle ground track of substantiallyflat terrain having random and different features, video signalprocessor means connected to the altimeter means for receiving the echosignal from each pulse-impacted point of the series, the echo signal ofa given pulse-impacted point being indicative of the average weightedreflectivity of one or more random and different terrain featuressurrounding the given point, said video signal processor means beingcomprised of logarithmic amplifier means, comparator means, delay linemeans, and track-and-hold means, said comparator means and saidtrack-and-hold means being parallel connected to the output of saidlogarithmic amplifier means, said delay line means being connected tothe output of the comparator means, said track and hold means beingconnected to the output of the delay line means, said track and holdmeans for receiving and correlating the outputs of the amplifer meansand the delay line means in order to determine a point of intersectiontherebetween that is indicative of a qunatitized value of a delay sampleof an annulus made up of one or more random and different terrainfeatures of average-weighted reflectivity of an echo signal about agiven pulse-impacted point along the vehicle ground track during systemuse, said video signal processor means being provided with furthermeans, said further means being connected to the output of the track andhold means, for transforming the delay sample of each annulus into adigital signal, and master processor means connected to the output ofthe further means for combining a series of digital signals of the videosignal processor means into an arrayed output and then comparing thearrayed output with a series of stored digital arrays arranged in matrixfashion in order to ascertain which stored digital array of the seriesthereof is substantially matched by the arrayed output for the purposeof enabling the determination whether the actual ground track of thevehicle corresponds to the intended ground track thereof.
 2. A system asset forth in claim 1 wherein said further means is analog-to-digitalconverter means.
 3. A system as set forth in claim 1 wherein each pulseis of compressed duration and of teardrop configuration.
 4. A system asset forth in claim 1 wherein said vehicle is a missile.
 5. A system asset forth in claim 1 wherein said further means is analog-to-digitalconverter means.
 6. A system as set forth in claim 1 wherein registermeans and digital-to-analog converter means are series interconnectedbetween said master processor means and said comparator means, saidmaster processor means together with said register means and saiddigital-to-analog converter means all cooperating to determine athreshold bias level output to the comparator means for the purpose ofremoving background noise from an echo signal during system use.
 7. Asystem as set forth in claim 1 wherein said video signal processor meansincludes Doppler filter means.
 8. A radar guidance system for thenavigation apparatus of an airborne vehicle, said systemcomprising:master processor means, radar altimeter means connected to anoutput of the processor means and operating in response to the output ofthe processor means for transmitting a series of pulses for impacting aseries of relatively spaced points along the vehicle ground track ofsubstantially flat terrain and further operating to receive an echosignal from each pulse-impacted point of the series of spaced points,video signal processor means generally made up of logarithmic amplifiermeans, comparator means, delay line means and track and hold means, saidamplifier means being connected to the output of said altimeter meansand receiving from the output of the altimeter means the echo signal ofeach pulse-impacted point, the output of said amplifier means beingparallel connected to the positive input of the comparator means and thetrack and hold means, the track and hold means and the delay line meansbeing series interconnected to the output of the comparator means suchthat the track and hold means is connected to the output of the delayline means, the delay line means for delaying the leading edge of theoutput of the comparator means a predetermined amount of time, the trackand hold means for correlating the outputs of the amplifier means andthe delay line means so as to form a point of intersection between theoutputs, such intersection point being quantization of a delay sample ofan annulus of the echo signal about a given pulse-impacted point alongthe vehicle ground track where each delay sample is representative ofthe reflectivity of an annulus about a pulse-impacted point; and wherethe annulus is made up of one or more random and different terrainfeatures, said video signal processor means being comprised ofdigital-to-analog converter means and threshold register means, thedigital-to-analog converter means and the register means being seriesinterconnected such that the output of the digital-to-analog convertermeans is connected to the negative bias input of the comparator meansand the input of the register means is connected to another output ofthe master processor means, said register means including feedback meansconnected to the master processor means, the other output of said masterprocessor means together with the feedback means of said register meanscausing selective adjustment of the negative bias of said comparatormeans in order to determine the threshold noise level of each echosignal so as to substantially eliminate noise from the output of thecomparator means when a series of amplified echo signals are received bythe positive input of the comparator means as the result of a series ofpulses being transmitted from said altimeter means for impacting aseries of relatively spaced points along the vehicle ground track duringsystem use, and converter means connected to the output of the track andhold means for transforming the quantization of the delay sample of eachecho signal into a digital signal, said master processor means beingconnected to the output of the converter means for combining a series ofdigital signals into an array and comparing the digital signal arraywith a plurality of digital signal arrays as arranged in a referencematrix in order to ascertain which array of the plurality of matrixeddigital arrays is substantially matched by the digital array of thecombined series of signals of the delay samples so as to determinewhether the actual ground track of the vehicle corresponds to theintended ground track thereof.
 9. A system as set forth in claim 8wherein each pulse transmitted by said altimeter means is of compressedduration and teardrop configuration.
 10. A system as set forth in claim8 wherein said video signal processor means includes Doppler filtermeans.
 11. A method for determining the actual ground track of anairborne vehicle; said method comprising the steps of:transmitting aseries of electromagnetic pulses from the vehicle in a downwarddirection for impacting a plurality of preselected relatively spacedpoints along the vehicle ground track of relatively flat terrain, whereeach pulse of the series is transmitted at a predetermined time intervaland is directed toward its respective point of the plurality ofpreselected points along the vehicle ground track, and where the terrainhas different reflectivity characteristics by virtue of random anddifferent features therealong, receiving at the vehicle an echo signalfrom each pulse-impacted point of the series of pulse-impacted andrelatively spaced points along the vehicle ground track while at thesame time amplifying each received echo signal, dividing each amplifiedecho signal, comparing one of the divided signals of each amplified echosignal with a predetermined negative bias level for the purpose ofsubstantially eliminating background noise therefrom while at the sametime transforming the compared signal into a substantial constantoutput, delaying the leading edge of the constant output of the comparedsignal of each amplified echo signal a predetermined amount so that thedelayed edge is within the duration of the envelope of its associateddivided signal, correlating the divided signals of each amplified echosignal such that the delayed leading edge of the compared signal iscorrelated with its associated divided signal so as to determine foreach amplified echo signal an intersection point between its comparedsignal and its associated divided signal, the intersection point of eachamplified echo signal representing the quantitized value of a delaysample thereof where any delay sample of a given amplified echo signalthat stems from its associated pulse-impacted point along the groundtrack is indicative of the average weighted aggregate reflectivity of anannulus of one or more random and different terrain features about theassociated pulse-impacted point therealong, transforming the delaysample of each echo signal into a digital format, combining all of thedigital formats of the samples into a data array, and analyticallycorrelating the data array of the samples with reference matrix datahaving a series of data arrays for the purpose of substantially matchingthe data array of the sample with one data array of the series of dataarrays of the reference matrix data so as to enable the determination ofwhether the actual ground track of the vehicle corresponds to theintended ground track thereof.
 12. A method as set forth in claim 11wherein the step of comparing is effected by comparator means.
 13. Amethod as set forth in claim 11 wherein the combined steps ofamplifying, dividing, comparing, delaying, correlating and transformingis effected by video signal processor means.
 14. A method as set forthin claim 11 wherein the steps of combining and analytically correlatingis effected by master processor means.
 15. A method as set forth inclaim 11 wherein the step of delaying is effected by delay line means.16. A method as set forth in claim 11 wherein the step of correlating iseffected by track and hold means.
 17. A method as set forth in claim 11wherein the step of transforming is effected by analog-to-digital means.18. A method as set forth in claim 11 wherein the step of amplifying iseffected by logarithmic amplifier means.
 19. A method as set forth inclaim 11 wherein the steps of transmitting and receiving is effected byradar altimeter means.